Patent GB-330972-A (GB)
Title: Improvements relating to apparatus for use in connection with photographic aerial surveys
IPC: G01C11/00
Filed: 1929-03-21
Abstract: 330,972. Aldis, A. C. W. March 21, 1929. Photographic surveying.-Relates to means for ascertaining the amount of error due to the tilt of the camera when making aerial photographic surveys, so that a rectified photograph may be made by the apparatus described in Specification 329,383, and then used for the production of a map of the territory surveyed. The amount of the error is ascertained by viewing a rectified photograph, namely, one in which no error due to camera tilt exists, and an uncorrected photograph, in a stereoscope in conjunction with a pair of correlated screens one of which corresponds to a known angle of tilt. The other screen of the pair is a rectangularly ruled screen, the scale of which corresponds to the ratio of the heights at which the two photographs were taken. Use is made of a partial ground survey in which the boundaries of the area are surveyed in the ordinary manner, the data obtained being applied to the correction of the first of the series of photographs of the territory under survey. After the error due to tilt in one photograph has been ascertained as the result of the stereoscopic examination and a corrected photograph taken from the original photograph, the corrected photograph is used for ascertaining in the same manner the error due to tilt in the next succeeding photograph. The photographs to be compared are supported on a movable table t, rotatable in ring r, itself rotatable in a ring 4 mounted on a plate n supported on guides m carried on a frame f. This frame f is counterweighted and may be moved up and down pillars a carried by a base band supported by ties d as shown. The photographs may thus be brought into the focus of the binocular viewing instrument k, being illuminated from below by lamps 17. The photographs u are secured to the plate by slidable gripping pieces, two of which are adjustable by screws against springs in the remaining oppositely disposed gripping pieces. Rotatable adjustment of the photograph is effected by the worm v. Above each photograph u is mounted a ruled screen 9 carried in a ring 10 in a circular frame 11 having a rib 12 engaged by rollers 13 supported from a plate x. This plate x is supported from a plate 2 by rollers z in brackets y, the rollers moving in grooves on the plate 2 to permit of motion of translation of the screen with respect to the photograph. The screen 9 may be rotated by the milled head 21. The correct distance between the two photographs is maintained by the crossbar 22 secured to the rings r. The screens used consist of photographs of a ruled screen 33, Fig. 9, having a series of lines at right angles and marked with a pair of 'heavy lines across its centre. For that to be used with the rectified photograph the axis of the camera 36 is vertical end the distance of the nodal point 38 of the lens from the screen 33 is adjusted to give photographs of different scales, the distance of the point 38 from the plate 37 being the same as that of the nodal point of the camera lens from the plate in the camera making the survey. A series of photographs of the screen 33 is also made with the camera 36 at different angles of tilt, the centre 42 of the plate 37 being marked on each. The screens having zero inclination have the lines parallel and at right angles, as in dotted lines in Fig. 10 whereas the inclination screens have the lines, shown full in Fig. 10 diverging, the distortion varying with the degree of inclination. Two photographs, Fig. 12, forming a stereoscopic pair, are placed in the viewing apparatus, one of them, in the example, the lefthand one, being a rectified photograph or one in which there is no error due to camera tilt, and having a point 0 vertically below the camera lens. In the photograph to be compared, the right hand one, the point A corresponds to the position of the camera axis. A zero inclination screen is used in conjunction with the photograph having the point 0 and various inclination screens are used with the other photograph until one is found which is seen stereoscopically in conjunction with the first screen as a single screen superimposed on the ground. If combination cannot be effected in this way a different scale screen is used with the rectified photograph suitably related to the relative heights of the two photographs. When this result is achieved the point 42, Fig. 10, coincides with the point A<1> in Fig. 12, the position of the point 41 indicating the plumb point P<1> which it is desired to find. This point is marked on the photograph with the aid of a jig placed in the position occupied by the ruled screen, a hole in the jig being adjustable to correspond to the distance between the points A<1>, P<1>. The point is marked by a drill or the like inserted through the hole. The photograph is then used in the correcting apparatus described in Specification 329,383 to obtain a photograph which can then be used for determining the correct position of the plumb point of the next photograph and similarly throughout the whole range.